Lithography is employed in semiconductor manufacturing to pattern features in a photoresist. A layer of photoresist is exposed to incident light, which may be deep-ultraviolet (DUV) radiation, mid-ultraviolet (MUV) radiation, or X-ray radiation. Alternately, the layer of photoresist may be exposed to energetic electron in e-beam lithography. The energy of the photons or electrons causes changes in chemical composition of exposed portions of the photoresist, for example, by cross-linkage, scission, side chain removal, etc. Pre-baking or post-baking of the photoresist may be employed to maximize the changes in the chemical properties of the exposed portion of the photoresist relative to unexposed portions of the photoresist.
The exposed photoresist is developed to remove one of the set of exposed portions of the photoresist and the set of unexposed portions of the photoresist relative to the other. The photoresist is classified as a positive photoresist or a negative photoresist depending on the nature of the chemical changes upon exposure. If the photoresist becomes chemically less stable upon exposure, the photoresist is a positive photoresist. If the photoresist becomes chemically more stable upon exposure, the photoresist is a negative photoresist. In case a positive photoresist is employed, the exposed portions of the positive photoresist are removed upon development. In case a negative photoresist is employed, the unexposed portions of the negative photoresist are removed upon development.
A developed photoresist comprises a lithographic pattern. The features of the lithographic pattern have dimensions that are the same as, or greater than, a “minimum feature size,” which is also called a “critical dimension.” The minimum feature size is a function of a lithography tool employed to form the lithographic pattern. The minimum feature size F that a projection system can print is given approximately by:F=β×λ/NA,where β is a coefficient, or a process prefactor, that reflects tool specific efficiency of the lithography system and other process related factors, λ is the wavelength of the light employed for radiation, and NA is the numerical aperture of the lens. Typically, the value of the coefficient β is in the range of about 0.5.
While the minimum feature size is defined only in relation to a lithography tool and normally changes from generation to generation of semiconductor technology, it is understood that the minimum feature size, i.e., the critical dimension, is to be defined in relation to the best performance of lithography tools available at the time of semiconductor manufacturing. As of 2008, the minimum feature size for a line width is about 45 nm, and is expected to shrink in the future. Any dimension less than the lithographic minimum dimension is called a “sublithographic dimension.” Any dimension equal to or greater than the minimum feature size is a lithographic dimension.
A pitch of a line and space structure includes the width of the line and the spacing of the space. The lithographic minimum pitch for a line and space structure, or the minimum pitch for the line and space structure that may be formed with a single layer of photoresist, a single exposure, and a single development, is about 100 nm as of 2008, and is expected to shrink in the future.
While developments are under way to provide high numerical aperture exposure systems and/or increase the coefficient β to minimum feature size F, the rate of reduction of the minimum feature size F through such efforts is far slower than the rate of reduction of desired feature sizes for high performance of semiconductor devices. Further, obtaining light sources that provide reduced wavelength in the deep ultraviolet (DUV) or extreme ultraviolet (EUV) range turns out to be a difficult challenge. Currently, an adequate light source to enable printing of dimensions less than 45 nm is not commercially available.
The pattern in the developed photoresist is subsequently transferred into an underlying layer employing the developed photoresist and an etch mask. Thus, the dimensions of features that may be formed in a semiconductor structure are directly tied to the dimensions of features in the developed photoresist. The minimum pitch of a repetitive lithographic pattern is the twice the minimum feature size since each unit pattern includes a line and a space or a via hole and a surrounding spacer.
Prior art lithographic methods thus face a fundamental limitation in the pitch of the pattern that may be printed once the wavelength of a lithographic system is given. In other words, a minimum pitch is determined more or less by the wavelength of the lithographic system.
In view of the above, there exists a need to provide methods of forming features having a pitch that is less than a conventionally determined minimum pitch of a lithographic system.
Specifically, there exists a need to provide methods of forming features in which the pitch of the pattern is reduced below twice the minimum feature size.